The present invention relates generally to a device for controlling the travelling of a printing head in a printer of the type in which the printing head is caused to travel in reciprocal directions perpendicular to the feeding direction of a recording medium, and more particularly, to a device for controlling the travelling of a printing head in a printer of the type in which the printing head is caused to reciprocate by a lead screw having a helical guide groove formed on its outer periphery.
A printer in which a printing head is caused to travel by a lead screw is known. In this known printer, the lead screw is provided on its outer periphery with a single helical guide groove and is forward/backward driven by an expensive stepping motor to cause the printing head to reciprocate along its travelling path. Another printer is also known in which a printing head is caused to reciprocate by a lead screw having a first guide groove for causing the printing head to move in the forward direction and a second guide groove for causing the printing head to move in the backward direction, the first and second guide grooves being wound in opposite directions to each other to intersect each other and being connected to each other at their opposite ends to form a closed loop. The first and second guide grooves both have the same pitch. The use of such a lead screw provides the advantage that the printing head can be reciprocated only by continuously rotating the lead screw in the same direction, so that an inexpensive motor can be employed. Although each of the prior art techniques as described above is suitable for a printer in which printing is made during each of the forward and backward travelling strokes, it is not suitable for a printer in which printing is made only during the forward travelling stroke for the following reason. That is, in each of the abovementioned two conventional printers, the time required for the backward travelling stroke in which no printing is made is equal to the time required for the forward travelling stroke in which printing is performed, so that the former is unnecessarily long and thus increases the time for the printing head to travel in one reciprocating cycle, thereby resulting in a reduction in the actual printing speed.
Accordingly, in a printer which performs a printing operation only during the forward travelling stroke of a printing head, in order to substantially increase the printing speed, there has been proposed an arrangement wherein the pitch guide groove of a for effecting the backward travelling stroke of the printing head is selected to be several times (for example twice) as large as that of another guide groove for effecting the forward travelling stroke of the printing head, the lead screw being continuously rotated in a fixed direction. By using such an improved lead screw, the time required for the printing head to return is shortened, resulting in a reduction in the time required for the printing head to complete one reciprocating cycle. However, this reduction in the time required for the printing head to make one reciprocating cycle, i.e. the increase in actual printing speed, is still insufficient. In printing data in a printer, there are a few cases where full characters are printed in each character line, while in many cases characters which are half or less in number in comparison with the full characters are printed in each character line. Even in the case where the last mentioned system is employed, the time required for the printing head to make one reciprocating cycle is constant regardless of the number of characters to be printed in the cycle, and consequently the actual printing speed is inferior to that of a printer of the type in which a printing head is returned by a return spring.